For example, at the front end of a cap body or the back end of a writing instrument body, an endcap which has a function as decoration of the writing instrument or a function as a pedestal for a clip is attached (Japanese Patent Publication No. 2009-214515A, Japanese Patent Publication No. 2010-247405A, and Japanese Patent Publication No. 11-221997A). For example, in Japanese Patent Publication No. 2009-214515A, an endcap is attached to a projection which is arranged in a restricting wall part which is formed at a top end of a cylinder. In attachment, an inside circumferential surface of a mounting hole of the endcap is fit with an outside circumferential surface of the projection. The endcap is attached to the cylinder by a predetermined mounting strength in this way.